Because cryptocurrencies are dependent on the reliability of distributed systems and cryptography. Both concepts are extremely tricky, the only true way of validating them is by trying to break them and failing, this takes time. But the floodgates have been opened.
Also, who cares if there are several cryptocurrencies? If they have good properties (fundamentals) and people find them valuable, why not?
Software isn't some magic that can wish away its shorcomings.
Also, do you think the next instagram that will most like support its own crypto will allow you to switch to a different crypto?
ETA: the transaction cost is based on the cryptocurrency, so if now the cost per transaction is 1 cent then in 10 years it might cost 10 dollars if the value of the coin goes up.